The present invention pertains to processes for forming a photoresist mask having dimensions that are less than the dimensions of a pattern projected onto the photoresist to form the mask, and to processes using such a mask.
Semiconductor integrated circuits (ICs) are composed of devices such as transistors. While conventional fabrication processes are capable of providing millions of devices in a single IC, there is a need to decrease the size of the device features so that the number of devices in an IC may be further increased.
One limitation on the size reduction of IC device dimensions is the resolution capability of conventional projection lithography. In general, projection lithography is a process by which a pattern is formed during fabrication of an integrated circuit. Such patterns are typically used to form device structures such as doping regions, deposition regions, etching regions, and other structures. Thus the device dimensions are limited by the size of the patterns that can be projected.
In conventional projection lithography, a substrate is coated with a radiation-sensitive photoresist. The photoresist is exposed to radiation that is projected through a reticle containing a pattern to be formed on the substrate. Various types of radiation are conventionally used such as ultra-violet light, vacuum ultra-violet (VUV) light, deep ultra violet light, x-ray radiation, and e-beam radiation. Exposure of the photoresist to the radiation causes the exposed area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. As a result, the photoresist receives the pattern of radiation that is formed by projection through the reticle. The unwanted areas of the pattern are then removed in a developing process. Conventional projection lithography processes are either xe2x80x9cbright fieldxe2x80x9d processes, in which areas to be removed are exposed to radiation, or xe2x80x9cdark fieldxe2x80x9d processes, in which areas to be retained are exposed to radiation. A processing step such as etching, diffusion, implantation or deposition is then performed using the photoresist pattern to selectively prevent the effects of the processing step. The remaining photoresist is subsequently removed in a stripping process.
The resolution capability of projection lithography is limited by the wavelength of the radiation used to project a pattern onto the photoresist. Therefore one approach to increasing resolution is to reduce the wavelength of the radiation that is used. Presently, extreme ultraviolet (EUV) lithography enables smaller feature sizes by using short wavelength xe2x80x9clongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays having wavelengths in the range of 50 to 700 Angstroms. However, the use of shorter wavelengths requires the development of new photoresist chemistries that are responsive to those wavelengths. Therefore the feature sizes achievable through conventional projection lithography systems remain limited. Currently the smallest achievable gate width is approximately 0.18 xcexcm. Accordingly, there is a need for processes that enable the formation of smaller structures using conventional projection lithography wavelengths and chemistries.
Conventional integrated circuit fabrication techniques may employ a process known as silyation. Generally, silyation involves the introduction of silicon into a photoresist material. Conventional techniques use silyation in combination with projection lithography in a process called a dual multilayer resist process. In this process, a thick layer of photoresist is applied over a non-uniform substrate to provide a planar photoresist surface. A pattern is then projected onto the photoresist, causing exposure of an upper portion of the photoresist layer. This is referred to as top surface imaging (TSI). Depending on the photoresist chemistry, the exposure increases or decreases the permeability of the photoresist to silicon in the exposed area. A silyation process is then performed to selectively introduce silicon into the permeable areas of the photoresist. The photoresist is then exposed to an anisotropic oxygen plasma etch, which converts the silyated photoresist to etch resistant silicon dioxide while simultaneously etching the photoresist that does not lie beneath the silyated photoresist areas. This technique allows the formation of a planar surface through application of a thick layer of photoresist to avoid distortions that would be caused by projection onto a nonplanar layer, without introducing the optical distortions that would result from exposure of the entire thickness of the photoresist.
Various chemistries for silyation of photoresist are known in the art. Silyation agents may be supplied as a gas in a dry silyation method, such as is described in U.S. Pat. No. 5,562,801, or may be supplied as a liquid in a wet chemistry method, such as is described in U.S. Pat. No. 6,063,543, each of which is incorporated herein by reference for these teachings.
While silyation has been used to minimize defects in patterns formed in a photoresist layer by projection lithography, conventional technology has not employed silyation for the purpose of reducing feature sizes in patterns formed in a photoresist layer by projection lithography.
In accordance with embodiments of the invention, a pattern is projected onto a photoresist to form an area that is then silyated. The photoresist is then etched to remove photoresist that does not underlie the silyated area, and to undercut the photoresist beneath the silyated area so that the features of the photoresist beneath the silyated area are narrower than the corresponding features of the silyated area. Thus the resulting photoresist pattern has narrower features than those that were projected onto the photoresist.
Embodiments of the invention relate to a process for forming a photoresist mask. A photoresist layer is applied to a substrate. A silyated layer is selectively formed in the photoresist layer. The features of the silyated area correspond to the features of a photoresist mask to be formed. The photoresist layer is then selectively etched to form a photoresist base beneath the silyated area. The photoresist base is narrower than the silyated area. The silyated area is then removed, leaving a photoresist mask on the substrate.